A self-assisting rim brake is disclosed in French Patent Specification 900,660. In this known rim brake, two arms of a brake caliper are pivotable about a common pivot pin and tiltable about an axis which is approximately at a right angle to the axis of the pivot pin. Rollers are provided on the arms for engaging sloping faces on fixed supports adjacent each side of the wheel so as to provide additional braking force when the brakes are applied. However, in this arrangement, the arms have bores overlying the pivot pin which must be substantially oversized to provide the play necessary to allow the arms to tilt. This play also allows for unsynchronous pivot movement of the arms during operation. Furthermore, the sloping faces and the rollers are arranged level with the brake shoes. As a result, the length of the lever arms between the pivot pin and the brake shoes is approximately equal to the lever arm between the pivot pin and the sloping faces or the rollers. Therefore, the dimension of the restoring spring for bringing the brake shoes into their initial position is very difficult to determine. If the spring is made too weak, the brake remains in its braking position and the brake shoes will wedge between the sloping faces and remain wedged until the direction of rotation of the braked rim is reversed. Clearly this practice is impractical if not virtually impossible. In contrast, if the restoring spring is made too strong, the braking force exerted by the operator becomes higher and the assisting effect of the sloping faces is also impaired.
The same problems exist in the rim brake which is disclosed in German Patent Specification 1,116,558. In this arrangement, the arms of the brake caliper are pivotable about a common pin and additionally are displaceable along the pin against an elastic compression bumper positioned on the pin. The two brake shoes include a holder in which a brake block is anchored. The holders include a sloping face formed at one end which engage the spars of the bicycle frame. When the brake blocks are pressed against the rim, the arms of the brake caliper tend to shift along the pin against the elastic compression bumper. The result of this displacement is that the sloping faces of the holders engage the frame spars and the brake shoes are pressed firmly against the rim. According to the design, when the brake is released, a restoring spring is supposed to push the arms of the brake caliper back into the initial position. In practice, however, the dimensioning of the restoring spring presents problems. If it is too week, the arms of the brake caliper wedge between the spars of the frame locking the wheel. The wheel even remains locked after the brake is released. In contrast, if the restoring spring is made too strong, the self-assisting effect is substantially reduced if it occurs at all. In addition, the self-assisting effect is also dependent on the friction between the sloping faces and the spars. The friction is dependent on, among other things, the weather conditions and any soil buildup, therefore the braking force will be inconsistent.
Accordingly it is an object of the present invention to provide an improved self-assisting rim brake which avoids the drawbacks of the prior art as discussed above.
It is a more particular object of the present invention to provide an improved rim brake of the type mentioned in the introduction, in such a way that the self-assisting effect is virtually independent of the dimensioning of the spring and is substantially consistent under all weather conditions.